Use of sound waves, specifically ultrasonic sound waves for determining fluid density is well known. An ultrasonic sensor for measuring fluid density was reported by S. Sheen at Argonne National Laboratory. It received a R&D 102 Award in 1994 and a description appeared in the Research and Development magazine in October, 1994, p. 15.
Sheen describes an ultrasonic densitometer (FIG. 1) for measuring a density of a fluid 100. The ultrasonic densitometer has a wedge material 102 wherein the wedge material 102 has at least two sides substantially parallel. A first parallel side 104 has a first ultrasonic transducer 106 mounted thereon and a second parallel side 108 immersible into said fluid whereby a first portion of an ultrasonic signal emanating from said first ultrasonic transducer 106 strikes the second parallel side 108 and reflects back to the first parallel side 104 providing a reflection coefficient. A second portion of the ultrasonic signal propagates through the fluid 100, strikes a second wedge immersed surface 110 and reflects back to the first ultrasonic transducer 106 providing a speed of sound in the fluid. The arm surfaces 112 of the T are in contact with air for reference measurements. The T-shaped wedges 102 are mounted through the wall of a pipe 114 so that the fluid 100 within the pipe passes between the immersed surfaces 108, 110 of the two T-shaped wedges 102, 102a. From the reflection coefficient and the speed of sound in the fluid, the density of the fluid is obtained. In a second paper S. H. Sheen, H. T. Chien, and A. C. Raptis, "An In-Line Ultrasonic Viscometer," Review of Progress in Quantitative Nondestructive Evaluation, Vol. 14a, pp 1151-1158, 1995, Sheen specifies that the T-shaped wedge material is aluminum. The second transducer generates shear waves used for determining viscosity. A disadvantage of Sheen's ultrasonic densitometer is that because the wedge material is aluminum, the acoustic impedance of the wedge material is much greater than the acoustic impedance of the fluid so that a substantial change in density (eg. 10%) results in a quite small change in the aluminum/liquid reflection coefficient of about 0.014. Secondarily, the ultrasonic signal is required to reflect through the fluid of interest thereby requiring the requisite target surface of a second T-shaped wedge. Further, for fluids attenuative of ultrasound, density measurements would not be obtainable.
Another ultrasonic fluid meter is described in M. S. Greenwood, J. L. Mai, and M. S. Good, "Attenuation measurement of ultrasound in a kaolin-water slurry: A linear dependence upon frequency," A J. Acoust. Soc. Am. 94, 908-916 (1993). This ultrasonic attenuation sensor was developed for concentration measurements in a 1/12-scale model of a double-shell tank. Because fluid density is a function of concentration, this unit may be used to determine fluid density as well as fluid concentration. The sensor consists of a send transducer and a receive transducer, separated by 4 inches. The ultrasound produced by the send transducer travels through a liquid (or slurry) where it is attenuated. The signal recorded by the receive transducer indicates how much attenuation has occurred. However, the instrument required calibration by making measurements in the laboratory for that specific slurry formulation so that concentration of the slurry could be correlated with voltage of signal in receive transducer. Again, this ultrasonic densitometer required that the ultrasonic signal be detected after passing through the fluid, in this case slurry, of interest and further required prior laboratory calibration.
Commercially available ultrasonic fluid concentration measuring devices are available through JM Science Inc, Buffalo, N.Y., Manufactured by: Fuji Ultrasonic Engineering Co., Ltd. In operation, an ultrasonic transducer produces ultrasound that propagates through the fluid of interest then is reflected by a metal plate about an inch away from the transducer. The reflected signal returns to the transducer and the time for a round trip is determined. Since the distance is known, the velocity of ultrasound in the liquid can be determined. The Fuji sensor correlates the speed of sound with a concentration of a particular fluid solution and with temperature of the particular fluid solution and requires laboratory calibration. As with Greenwood et al., the reflected ultrasonic signal must pass through the fluid of interest and the instrument requires calibration.
There is a need in the field of ultrasonic densitometry for an ultrasonic fluid densitometer that has greater sensitivity, does not require calibration and does not require a reflected signal to pass through the fluid of interest.